1. Technical Field
The present invention relates generally to an analog-to-digital converter for converting various image signals to digital signals and, more particularly, to an analog-to-digital converter that prevents signal distortion during gamma correction and which-simplifies digital image processing.
2. Description of Related Technology
When analog image input signals are expressed on a display apparatus, such as a CRT monitor, original colors may be distorted because the display apparatus typically has a non-linear response characteristic to input signals. Such a non-linear response characteristic is shown as a dashed line in FIG. 1.
To correct distortion and display the original image input signal, a gamma correction is typically performed. A gamma correction corrects a distorted image signal by using a gamma function, which is depicted as a solid line in FIG. 1. Gamma correction is an important image processing operation because the image quality of the display apparatus may be dependent on how the gamma correction is applied. Because conventional gamma correction is carried out after analog-to-digital conversion of the original input signal, distortion of the original signal during analog-to-digital conversion may render any subsequent gamma correction ineffective.
FIG. 2 is a block diagram that depicts one known image signal processing technique. As shown in FIG. 2, a conventional gamma correction technique includes the steps of converting input image signals (analog image signals) to digital signals by using an analog-to-digital converter at block 20, applying gamma correction to the digital signals at block 21 and performing image signal processing and sending the image signals to a display apparatus after gamma correction at block 22.
In the analog-to-digital conversion of the image input signals, when the input analog image signals do not exactly match with the digital signals because of the nonlinear characteristic of the analog signals, the mapping of the analog signals to the digital signals is performed using approximate digital values. One result of such an approximate mapping procedure is that the video input signals may be randomly distorted. Such random distortion is particularly problematic in the case of low level input signals.
Because conventional gamma correction is performed on a randomly distorted signal, an exact gamma function cannot be applied to the input signals. .Additionally, another digital processing block is required to perform the gamma correction after the analog-to-digital conversion of the input signals. In additional digital processing, because the digital processing converts a non-continuous input signal to non-continuous output signal by using a non-linear function, it is very difficult to implement the digital processing block of the gamma correction with simple multipliers. As a result, a larger chip, which consumes more power, is typically required to implement the digital processing block.
FIG. 3 is a block diagram illustrating another known image signal processing technique. As shown in FIG. 3, analog input signals are corrected at block 30 and the corrected analog input signals are converted into digital signals according to linear reference voltage at block 40 and then digital codes are output. To carry out this analog-to-digital conversion technique, an additional circuit is needed to cause the time-dependant input signals to have an exact transfer function. However, it is very difficult to implement such an additional circuit because of a distortion difference between levels of the input signals, a distortion of the transfer function according to frequency characteristic of the input signals and a transfer characteristic according to the input level of the analog signals.
In accordance with one aspect an analog-to-digital converter may include a gamma correction unit for generating a plurality of reference voltages corrected according to a gamma function and a decoding unit for selecting at least one corrected reference voltage in response to an input signal and performing analog-to-digital conversion of the selected reference voltage.